Various types of processing equipment have tanks provided with an intermittently rotatable, element-supporting shaft. For example, the centrifugal filter disclosed in U.S. Pat. No. 3,662,894 has a filter-supporting shaft which is rotated only when the filters are cleaned.
In equipment of this construction, it is desirable to provide a seal between the shaft and the tank wall to prevent leakage from the tank. However, such seals may be subject to extreme wear, particularly when corrosive or abrasive material is processed.
One attempt to increase the service life of seals in the type of equipment described above, is disclosed in U.S. Pat. No. 4,013,297. There, a conventional static seal is established between the shaft and tank during such times as the shaft remains at rest; however, when it is desired to rotate the shaft, the seals are released by fluid under pressure to an open mode such that surface-to-surface wear of the seal is eliminated. While the seal of this reference has proved successful in greatly reducing seal wear in certain types of processing equipment, problems have developed with its use. The seal is expensive to fabricate and install due to the relatively large number of parts associated with the seal, and similarly, removal and replacement of the shaft is much more tedious than in equipment employing conventional seals. Moreover, in order to avoid injecting pressurized fluid into the shaft bearing, a separate high pressure seal must be provided.